Phytoplankton productivity in newly opened waters of the Western Arctic Ocean

Phytoplankton carbon and nitrogen production in newly opened waters of the Western Arctic Ocean were measured using a C-13-N-15 dual isotope tracer technique in summer 2008, when Arctic sea ice was at its second lowest extent since 1979. Daily integrated carbon-uptake rates were highest (675.8 +/- 931.3 mg C m(-2) d(-1)) in ice-free shelf regions of the Chukchi Sea. In deeper regions, the daily integrated carbon-uptake rates (73.5 +/- 45.0 mg C m(-2) d(-1)) in newly opened deep waters with adjacent sea ice cover (10-90%) were significantly higher than those (32.0 +/- 22.8 mg C m(-2) d(-1)) in ice-free deep waters. The average contributions of small cell-size phytoplankton (0.7-5 mu m) to total carbon-uptake rates were 19.8%, 62.8%, and 55.5% in shelf regions, ice-free deep waters, and newly opened waters, respectively, although the average biomass contributions of the small phytoplankton was 54.9-96.5% of total algal biomass in the three study areas. Daily total nitrogen-uptake rates were similar between newly opened deep waters and ice-free deep waters, although nitrate assimilation rates were higher in newly opened waters compared to ammonium uptake rates in ice-free waters. Lower carbon- and nitrate-assimilation rates in ice-free waters were due to strong stratification in the euphotic layer caused by melted sea ice, which limited nutrient availability to phytoplankton, although phytoplankton had greater light availability in ice-free waters than in newly opened waters. (C) 2011 Published by Elsevier Ltd.